Field of the Invention
The invention relates to a power supply for supplying a main electrical circuit during normal operation and an auxiliary electrical circuit during standby operation.
Modern electronic equipment such as, for example, PC, fax or TV equipment nowadays usually has a standby function. In the case of a TV set in the standby state, the set is "switched off" and can be switched back to normal operation through the use of a remote control, for example. In the case of a fax machine, the machine is in the ready-to-receive state and is switched on by an incoming fax signal, for example. Therefore, the different equipment states of "standby operation" or "normal operation" frequently need separate requirements of the power supply. During standby operation, the current consumption ought to be as small as possible. In particular, the only equipment functions that ought to be supplied in that case are those which are necessary to ensure fast readiness for use and which are required to identify a start signal such as, for example, an infrared signal of a remote control or a received signal of a fax modem. In the case of future PC machines, which should be switched on and off through a local area network (LAN), the auxiliary power supply must additionally have a high efficiency. With a maximum efficiency for the operating states of an item of equipment being taken into account, the power supply is usually realized nowadays in such a way that a main power supply is provided for a nominal/maximal load and an additional auxiliary power supply is provided for a minimal load as is required, for example, during standby operation of a PC machine.
With regard to technological/economical considerations and in order to fulfill existing national and international standards and regulations, the main power supplies in the future will contain not only a known DC--DC converter operating on the basis of pulse width modulation for the main power supply of the item of equipment but also, in many instances, a further AC-DC converter, which is a so-called PFC converter (PFC=Power Factor Correction). The PFC converter will serve to effect the prescribed improvement in the power factor, while the traditional pulse width modulation converter will perform the transformation and supply system isolation. The power factor is defined as the ratio between absorbed active power and absorbed apparent power. The power factor is ideally equal to 1. There are no reactive volt-amperes in that case. The PFC converter is typically constructed as a step-up converter for cost reasons and due to its high efficiency. Alternatively, the PFC converter may also be constructed as a flyback converter, for example.
It would be advantageous, then, to use the PFC converter as a power supply for standby operation with the item of equipment or pulse width modulation converter switched off, and thus to obviate the need for a separate auxiliary power supply. Such power supplies of a switched mode power supply unit are specified, for example, in an article entitled "Power-Factor-Controller TDA 4815/19 verbessert Leistungs-faktor von Schaltnetzteilen" [Power Factor Controller TDA 4815/19 Improves Power Factor of Switched Mode Power Supply Units] by Werner Schott, in Siemens Components 31 (1993), issue 2, pages 46 to 50. Furthermore, power supplies of the afore-mentioned type are described, for example, in data sheets concerning the TK 84 819 module from the company Toko, the LT 1509 module from the company Linear Technology, and the ML 4824 module from the company Microlinear. However, in those cases what is disadvantageous is that the operation of the PFC converter necessitates a base load, which must be greater as the input voltage and the output current become higher. As a result, only a very unfavorable efficiency is obtained.